Polymer modified asphalt for paving applications and methods for preparing various polymer modified asphalt compositions are well known in the art. Various polymers, including diene elastomers such as polybutadiene, EPDM, EPR and styrenic block copolymers (SBCs) such as styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene (SIS) block copolymers, are commonly combined with bitumen to improve the performance of the bitumen. See, for example, U.S. Pat. No. 5,190,998 and U.S. Pat. No. 6,150,439.
It is well known in the art to utilize styrenic block copolymers to improve the performance of bitumen. The degree to which property improvements are realized depends upon the compatibility of the block copolymer and the bitumen. Over the years, researchers have developed a range of chemical techniques to improve the compatibility of polymers with bitumens. An extensive range of additives (sulfur crosslinking agents and sulfur-containing crosslinking agents) have been used for the purpose of “curing” or “crosslinking” the polymers and bitumens thereby rendering the mixture compatible. Sulfur has long been known to be added to polymer modified bitumens to strengthen and accelerate the polymer-bitumen bonding and curing process. See, for example, U.S. Pat. No. 4,145,322 and U.S. Pat. No. 4,242,246. While sulfur itself is relatively safe, the reaction between sulfur, the block copolymers and asphalt generates hydrogen sulfide (H2S) which is extremely hazardous. As a result, the elimination of sulfur and sulfur containing crosslinking agents which aid in the curing of bituminous formulations would be considered a positive aspect.
U.S. patent application Ser. No. 11/598,511, published as US 2007/0112102 A1, discloses a process for preparing a polymer modified bituminous binder composition in the substantial absence of crosslinking agents, using a block copolymer composition that contains a high vinyl content diblock copolymer and optionally a high vinyl content block copolymer selected from linear triblock copolymers, multiarm coupled block copolymers and mixtures thereof. Using such a process and composition results in a product that is storage stable, readily mixable and has a quick cure while at the same time minimizing the environmental impact caused by the addition of sulfur or sulfur-containing crosslinking agents.
In areas having moderately cold to severe winters, high ductility at low temperatures is also an extremely desirable property. Bituminous material that does not have sufficient ductility tends to produce spurious pavement surfaces and cracking in moderately cold or severe weather thereby leading to asphalt failure and poor to dangerous surface conditions.
Accordingly, there exists a need for an improved polymer-modified bituminous binder composition and process for preparing the same which demonstrates not only high ductility but also the properties achieved when utilizing the bituminous binder composition of US Publication 2007/0112102 A1. Ideally, such a process and product would provide ready mixing, be stable in storage, have a quick cure of polymer and bitumen and have improved ductility, particularly at low temperatures.